Parasitic flatworms that can cause liver and blood diseases in humans could become more common as global temperatures rise, according to a new study.

Palaeobiologists at the University of Missouri have found evidence in ancient shellfish fossils from around 9,600 years ago of high levels of parasite infestations.

These trematode flatworms spend part of their life cycle in molluscs like clams, snails and mussels before then infecting fish, and later fish-eating birds and mammals.

The scientists found that during the Holocene, when temperatures were about 2.5°C warmer than they are today and sea levels were higher, molluscs suffered from far greater levels of infection. The researchers studied pits left by parasites in fossilised clam shells like the one above from 9,600 years ago

The scientists found that during the Holocene, when temperatures were about 2.5°C warmer than they are today and sea levels were higher, molluscs suffered from far greater levels of infection.

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They warn that as global temperatures are due to increase by up to 4°C by the end of the century and sea levels are due to rise, it could mean these parasites could become a growing problem to human health.

CLIMATE CHANGE WILL LEAVE A SOUR TASTE IN OUR MOUTHS

Marine biologists have found that shellfish take on a sour flavour if they are reared in slightly acidified sea water.

They warn that as the planet's oceans grow more acidic, due to rising carbon dioxide levels, many of our favourite seafoods could become less appetising.

Climate change experts predict that over the next century, the acidity levels of the world's oceans could drop from pH8 to pH7.5.

In taste tests shrimp raised in the waters with a lower pH were 2.6 times more likely to be rated as the worst tasting, while those reared in the less acidic water were 3.4 times more likely to be judged the tastiest.

Dr John Huntley, a geological scientist at the University of MIssouri who led the study, said: 'When compared to documented rises in sea level more than 9,300 years ago, we found that we currently are creating conditions for an increase in trematodes in present-day estuarine environments.

'This could have harmful implications for both animal and human health, including many of the world's fisheries.'

Dr Huntley and his colleagues studied the fossilised shells of ancient clams collected from the Pearl River Delta in China.

They looked at the oval shaped pits that form in the clam shells as the clams grow around the parasites.

'However, infected clam shells develop oval-shaped pits where the clam grew around the parasite in order to keep it out.'

The mouth of an adult liver fluke, a type of trematode that can infect humans, magnified by 22 times. These parasites could become more common as temperatures warm and sea levels rise, according to new research

Dr Huntley compared the pits in shells from clams found in the Pearl River Delta in China and those from Italy. Experts warn that as global temperatures are due to increase by up to 4°C by the end of the century and sea levels are due to rise, it may mean parasites could become a growing problem to human health

The researchers then compared these findings to clams found in the Adriatic Sea off the coast of Italy.

Using data that includes highly detailed descriptions of climate change and radiocarbon dating, they noticed clam shells contained more pits in the fossils during times of sea level rise both in China and Italy, when esturies would have been flooded with brackish water.

At least 56 million people globally suffer from one or more food-borne trematode infections, according to the World Health Organisation. The flatworm Schistosoma mansoni (pictured),a blood fluke or trematode, causes swamp fever in humans

A graphic from the Centre for Disease Control that shows how many common trematode infections spread. Dr Huntley warned that rising sea levels and global temperatures due to climate change could result in a growth in the number of infections

The symptoms can include inflammation, fever and can even be fatal.

At least 56 million people globally suffer from one or more food-borne trematode infections, according to the World Health Organisation.

He said: 'The insights offered by our high-resolution Holocene data are that trematode-prevalence values in brackish settings will be significantly higher during the initial stage of rapid sea-level rise in the coming centuries and will significantly decrease by the maximum flooding stage.

'An increase in trematode prevalence will have significant implications for ecological and human health.'

The trematode flatworm Heterophyes heterophyes (above) can cause intestine problems in those it infects